In pressure sorters of this type, the fundamental problem is that when no suitable countermeasures are taken, the throughput of usable fiber suspension through the screen and into the accepts chamber is drastically reduced in that the screen openings or apertures are clogged on the inlet side of the screen by impurities contained in the fiber suspension to be prepared, but also by fiber conglomerations; additionally, during the operation of such pressure sorters, the fibers contained in the fiber suspension to be prepared are fundamentally inclined to form a fiber fleece on the screen inlet side by means of which a high throughput of usable fibers (long as well as short) desired per se through the screen openings into the accepts chamber is prevented and besides, at least in most cases, undesired fractioning of the fiber suspension is effected--this means a separation of the fiber content in the fiber suspension to be prepared into shorter and longer fibers, whereby such a fiber fleece prevents, in particular, longer fibers from passing through the screen and into the accepts chamber.
The most varied measures are found in the state of the art by means of which it was attempted to control all or a part of the precedingly stated problems, whereby in this connection it needs to be realized that in pressure sorters of the type described in the beginning, the screen openings are intended to be flushed back by means of the negative pressure pulses generated by the profiled elements, i.e. by generating underpressure phases in the supply chamber liquid is intended to be sucked back from the accepts chamber through the screen openings and into the supply chamber in order to flush out from the screen openings impurities and fiber conglomerations collected at the inlet side of the screen openings.
A first measure, which can be deduced from the state of the art, consists in the fact that the screen openings are designed such that they widen in the conveying direction (i.e. in the direction from the supply chamber to the accepts chamber) (see for example, U.S. Pat. No. 3,581,903), in order to decrease the danger of the screen openings clogging up.
In order to effect a backwashing of the screen openings as well as to prevent a fiber fleece resulting on the inlet side of the screen, another known pressure sorter (see for example, U.S. Pat. No. 4,276,159) was equipped with a rotor which has in the vicinity of the screen inlet side rotating cleaning vanes with airfoil-like profile in section vertical to the rotor axis for generating positive and negative pressure pulses as well as having its screen designed such that due to the widened screen openings on the inlet side, a "roughened" screen inlet side results in order to generate turbulences in the fiber suspension to be prepared at the inlet side of the screen and in its vicinity by means of the interaction of the rotating rotor vane with the fiber suspension located in the supply chamber and the inlet side of the screen profiled in this manner, these turbulences counteracting the formation of a fiber fleece on the screen inlet side.
Also the most varied suggestions for the construction of a rotor of a pressure sorter can be deduced from the state of the art, namely, especially with reference to the design of the profiled elements for generating positive and negative pressure pulses in the fiber suspension in the supply chamber and/in the accepts chamber of the pressure sorter. The previously described cleaning vanes, as can be deduced for example from U.S. Pat. No. 4,276,159, were customary for a long time, as well as strip-shaped profiled elements, which extend approximately parallel to the screen axis and which are attached to the peripheral wall of a circular cylindrical and hollow rotor body. Examples for such strip-shaped profiled elements, which are arranged at a considerable distance from each other in the circumferential direction of the rotor, can be deduced, e.g. from FIG. 3 of DE-PS 25 26 657 as well as FIG. 3 of U.S. Pat. No. 4,200,537; in this respect, the last-mentioned state of the art shows strip-shaped profiled elements with an approximately triangular cross section, which have a first flank lying in front in the direction of rotation and projecting in radial direction beyond the peripheral surface of the rotor body, i.e. extending approximately perpendicular to the peripheral surface of the rotor body and a second flank sloping down towards the back. The fiber suspension located in the supply chamber of the pressure sorter is accelerated in rotational direction by the vertical front flank and it generates, in addition, positive pressure pulses while negative pressure pulses are generated by the sloping second flank.
Further rotor forms result, for example, from DD-PS 129 814 as well as from the U.S. Pat. Nos. 3,912,622, 3,726,401 and 3,400,820, however, these known rotor forms are not of importance in relation to the invention to be discussed in the following.
Other known suggestions concern the problem that due to the cleaning vanes or strip-shaped profiled elements which are continuous along the screen in the direction of the screen axis, such pressure impulses are generated in the fiber suspension that these impulses are disturbingly noticeable in the breast box of a paper machine following the pressure sorter (thereby, an uneven fiber fleece can form on the wire web of a paper machine). The fundamental idea of the known solutions to this problem is to subdivide the profiled elements into several segments transversely to the screen axis and to attach these segments to the peripheral surface of a circular cylindrical rotor body in such an arrangement that the segments following one another in the direction of the screen or rotor axis are offset relative to each other in circumferential direction of the rotor. In this respect, the profiled element segments of an axial rotor section each form a row in circumferential direction of the rotor, whereby a gap is located between two respective segments following one another in circumferential direction of the rotor and the lengths of the profiled element segments and the gaps--measured in circumferential direction of the rotor--are dimensioned such and the mentioned offset was chosen such that--as seen in the direction of the screen or rotor axis--the profiled element segments of an axial rotor section cover the gaps between the profiled element segments of the adjacent axial rotor sections. An example of such a rotor design can be deduced from DE-PS 37 01 669 (see in particular, FIG. 3); in this known rotor, the front surfaces or first flanks of the profiled element segments lying in front in rotational direction are designed such that they have a concave, arcuate profile in section vertical to the rotor axis, this profile ascending at an angle or diagonally towards the back and in radial direction towards the outside from the peripheral surface of the circular cylindrical rotor body in the direction opposite to the direction of rotation in order to reduce the impact effects of the pressure pulsations generated by the profiled element segments (see column 1, lines 12-14 of DE-PS 37 01 669).
Ultimately, a pressure sorter of the type mentioned in the beginning is disclosed in U.S. Pat. No. 4,855,038 and EP-0 206 975-B corresponding with the latter, the rotor of which is designed as a drum-shaped hollow body, whereby the peripheral wall of the rotor body forms two profiled elements directly adjoining each other in circumferential direction of the rotor, each element having a vertical leading first flank lying in a plane of diameter of the rotor as well as a second flank adjoined to the first and sloped downwards in the direction opposite to the direction of rotation. Each of these profiled elements extends over the entire length of the rotor in the direction of the rotor or screen axis, so that this also applies to the leading first flanks of the profiled elements extending parallel to the rotor axis. In addition, this known pressure sorter has a circular cylindrical screen, its inlet side (also when leaving the screen openings out of consideration) not being smooth but on the contrary, being profiled. Significance and purpose of the design of the rotor and the inlet side of the screen of this known pressure sorter is to constantly expose each region of the screen either to a positive or a negative pressure impulse, to generate great turbulences in the fiber suspension located in the supply chamber of the pressure sorter on account of the vertical leading flanks of the profiled elements and the great acceleration of the fiber suspension effected thereby in the direction of rotation in connection with the profiled inlet side of the screen and finally, to suck back considerable quantities of liquid from the accepts chamber through the screen and into the supply chamber of the pressure sorter by means of the long sloping second flanks of the profiled elements in order to eliminate with certainty the formation of a fiber fleece on the inlet side of the screen by a combination of all these measures.